The present invention relates to a method of electroforming a stamper to be used for forming or replicating a plastic disc, particularly a high-density information recording disc such as a video disc or a digital audio disc, and an apparatus for use with electroforming thereof.
The plastic disc to be used in a system for recording information signals in the form of submicron-order unevenness or pits on the plastic disc, reading the pits in an optical manner and playing back the same is mainly produced by techniques of compression molding, sheet molding, 2P (photo-polymerization) method and injection molding. In any such molding processes, a high degree of accuracy is required of a mold or a stamper to be used for transferring the submicron order pits onto plastic materials.
The stamper is generally produced by an electroforming technique as described in the following. A resist or a thin film of metal or the like is formed on a glass plate which has an optically completely flat surface. The resist is cut by a laser or the like to form pits or tracking grooves according to information signals. Such an original glass plate as above obtained as a mother die is electroformed to obtain a so-called master plate on which the pits are inversely transferred. Because the original plate normally has no electroconductivity, an electro conductive thin film is formed on the original plate by silver mirror reaction, vacuum deposition, sputtering or the like at the start of the electroforming of the master plate, and then a nickel-electroforming process is carried out. Information signals on the original plate are directly replicated on a plastic disc by using the master plate as the stamper when the plastic disc is molded. The depth of the signal pits may be generally set to 1/2n (n is a positive integer) of a wave length of a reading laser beam. However, since the surface of the master plate is a coating of above-mentioned electroconductive layer, and it sometimes does not have enough strength sufficient to endure repeated use in molding, a peeling treatment of the master plate is used to obtain a so-called mother plate having an inversed unevenness thereon. Then the mother plate is treated by peeling electroformed, thus obtaining the stamper.
The stamper is required to have certain characteristics such as a uniformity of thickness in addition to a superior transferability and mechanical strength. Ununiformity of thickness of the stamper directly affects ununiformity of thickness of the plastic disc to be molded, thus resulting in hindrance to a device for optically reading the disc. Further, such an ununiformity of thickness causes a metal mold to be damaged. A normal thickness of the stamper is in the range of 0.2-0.5 mm, and the permissable error is within 5% thereof.
To obtain such a nickel stamper, the original plate is normally mounted on an inclined rotary disc and is disposed in opposed relation with a nickel anode box or bag. Then, it is immersed in an electrodeposition solution containing nickel sulfamate as a main component. Direct current is conducted to the solution with stirring, thus accomplishing electroforming.
Generally, in case of a disc-like original plate, a peripheral portion of the disc tends to become relatively thicker, resulting in influence to uniformity of thickness. This is considered to be due to the fact that a difference in current density is created between a central portion of the mother disc and an outer portion thereof in dependence on the conditions of rotation of the mother disc and diffusion of circulating liquid. Accordingly, in such a case as above, in order to render uniform the thickness of a nickel layer to be formed by electrodeposition to the original disc, shape or size of an opening of the anode box or bag is generally adjusted and empirically set for the conditions to obtain an optimum uniformity of thickness in dependence upon an outer diameter of the original disc and the size of a central opening. In other words, conventionally the thickness of the stamper is adjusted by designing a shape of a shielding plate relative to an opening on the anode side to adjust the thickness of electroforming. However, since there exists a distance between an anode and a cathode, a shielding effect is not satisfactorily exhibited irrespective of provision of the shielding plate on the anode side, thus hindering an appropriate change in the thickness of electroforming. Further, if a large-sized shielding plate is used for increasing a shielding effect, current efficiency is disadvantageously decreased.